FIELD OF THE INVENTION
The present invention relates to a semiconductor device comprising a semiconductor substrate, an undoped semiconductor layer applied on the semiconductor substrate, an electron supply layer applied on the undoped semiconductor layer and having impurity atoms doped therein, and a gate metal layer applied on the electron supply layer.
In the field of the electronics, there has been required a semiconductor device having a high operation speed for use in a wide band amplifier and a high speed computing or calculating circuit. There have been proposed various techniques for attaining the high speed semiconductor device. For instance, in a field effect transistor (FET) or a bipolar transistor, it has been known to decrease the transit time by shortening a gate length or base width. There has been further proposed to increase the electron mobility in FET by using a so-called modulation-doping.
In case of decreasing the transit time by shortening the gate length of FET, the transistor operation is carried out by decreasing a distance over which the electrons transit, i.e. a distance between a source region and a drain region. However, this solution has a limitation that the gate length could not be shortened to such an extent that the function of the gate could no more be attained. Moreover, a manufacturing technique for shortening the distance between the source region and the drain region has also a limitation. The above mentioned modulation-doping method has been developed in order to overcome such drawbacks.
The modulation-doping method has been described in L. Esaki and T. Tsu: Superlattice and negative differential conductivity in semiconductors, IBM J. Res. Develop., 14, 61 (1970). In this modulation-doping method, the operation speed of a transistor is made higher by suppressing the scattering by impurity atoms which is one of major factors for determining the electron mobility. Said impurity atoms are necessary for providing electrons in a device. In this case, the impurity atoms are collected near a potential barrier, and thus the impurity atoms are separated spatially from a region in which the electrons travel by providing a spacer layer so that the electrons are hardly scattered by the impurity atoms. In this manner, a carrier concentration of the impurity atoms can be increased without increasing the scattering due to the impurity atoms.
The effectiveness of the above mentioned modulation-doping method has been experimentally confirmed by R. Dingle, H. L. Stormer, A. C. Gossard and W. Eiegmann: Electron mobilities in modulation-doped semiconductor hetero junction superlattices: Appl. Phys. Lett., 33, 665 (1978). Further, in Mimura, S. Hiyamizu, T. Fujii and K. Nanbu: Jpn. Appl. Phys., 19, L225 (1980), there is described the application of the modulation-doping method to a high electron mobility transistor (HEMT).
In H. Sakaki: Scattering suppression and high-mobility effect of quantum-confined electrons in ultrafine semiconductor wire structures: Jpn. J. Appl. Phys., 19, L735 (1990), there is further proposed a method for suppressing the impurity atom scattering by decreasing an electron conduction channel to such a size which is comparable to a wavelength of an electron wave. In this solution, quantum wire structure having ultrafine size is formed in a semiconductor crystal such that the electrons passing through the quantum wire structure are liable not to be subjected to the impurity atom scattering. In this method, the scattering probability is greatly reduced due to the fact that when the conduction channel is formed by a single mode electron wave guide, the mode coupling to a reflecting direction mode is caused by the scattering, and in order to scatter the electrons into the reflecting direction, it is necessary to cause a large change in the momentum energy. This method has been proposed in accordance with the effect based on the quantum wave property of electrons.
In the above explained modulation-doping method, in order to realize a high electron mobility it is inevitable to provide the spacer layer between the conduction channel and the impurity layer serving as the electron supply layer such that the conduction channel and impurity layer are separated from each other by a sufficiently long distance, so that the scatter suppressing function is increased when the thickness of the spacer layer is increased.
In the field effect transistor, when the modulation-doped structure is provided between the source and drain regions, upon controlling the electron concentration near the hetero junction by applying a voltage (charge) to the gate electrode, the gate electrode and hetero junction may be considered to constitute a flat plate capacitance, so that it may be interpreted that charges having opposite polarity are stored at the opposing hetero junction. In this case, there is generated a so-called edge effect. That is to say, electric force lines generated from the gate electrode are diverged at the edge of the gate electrode, and therefore all the electric force lines are not terminated at the two dimensional electron gas (2DEG), but are ended at another portions. This edge effect results in that only a part of the charges applied to the gate electrode contributes the transistor operation, and thus it is necessary to increase an amount of the charges in order to attain a desired transistor operation. This results in the increase in the operation time of the transistor.
The edge effect is manifest when a ratio of the gate length to a distance between the gate and the hetero junction is made smaller, so that if the operation speed of the transistor is increased by reducing the gate length, it is necessary to shorten the distance between the gate and the hetero junction. However, as stated above the spacer layer has to be provided, and thus the distance between the gate and the hetero junction could not be made smaller than a thickness of the spacer layer. In this manner, the suppression of the impurity atom scattering and the shortening of the distance between the gate and the hetero junction are conflict with each other and the modulation-doping method has a limitation in increasing the operation speed of the transistor.
In the above mentioned quantum wire method, use is made of the single mode transmission, and thus the scattering can be suppressed. However, a current passing through the quantum wire structure is limited. That is to say, in the single mode transmission path, an amount of the current which can be carried by electron, i.e. Fermi particle is limited to about 80 .mu.A per 1 eV and the conductivity of the channel is about 80 .mu.mho. Therefore, in order to conduct a large amount of the current, it is necessary to increase the conductivity of the channel. Moreover, if the quantum wire structure is practically applied to the semiconductor device, it is necessary to develop a fine crystal technique and a manufacturing method for forming the ultrafine structure having a dimension nearly equal to a wavelength of an electron wave in a crystal grown surface.